We propose to commercialize a proven, highly-parallel technology for chemical genomics in yeast. The technology is called Haplolnsufficiency Profiling, or HIP. Heterozygous deletion strains of Saccharomyces cerevisiae grow more slowly in the presence of a compound that specifically inhibits the gene product of the heterozygous locus. A set of approximately 6,000 strains representing deletions in all the genes in the yeast genome are grown together in a pool, and their relative growth rates are an indicator of relative fitness in the presence or absence of drug. Each deletion strain is tagged with a different oligonucleotide sequence, so that the responses of all approximately 6,000 strains can be read out simultaneously on a single microarray. As a result, the HIP assay can be used to interrogate the mechanism of action of medically relevant compounds in a comprehensive and unbiased fashion. In the Phase I proposal we focus on improvements to the readout portion of the assay. The completion of the aims will lay the foundation for building a complete, highperformance high-throughput small molecule profiling instrument suite. This powerful and cost-effective chemical genomics technology has the potential to play a significant role in drug development and basic research. Traditional approaches to drug development are inefficient, and there is a pressing human health need for new information-rich technologies that can greatly improve the process of identifying and validating targets and developing drugs.